This invention relates to transmissions for bicycles and, in particular, to an automatic transmission which can be attached within the rear fork of a bicycle.
Beginning with a two wheeled "hobby-horse" in 1816, the bicycle has gone through a long series of changes to make it more comfortable and easier to ride. In 1836, the hobby-horse received pedals attached to the front wheel. The desire for greater speed led to increasing the size of the front wheel, producing the high-wheelers ("ordinarys") of the late 1800's. The front wheel of an ordinary had a diameter as large as sixty inches, making the bicycle unstable and dangerous to ride.
In 1885, the "safety bicycle" introduced chain drive to the rear wheel in a gear ratio that produced as great a speed as an ordinary but on a much more stable bicycle. In 1888, Dr. Dunlop, a veterinarian, replaced the solid tires on his son's bicycle with handmade, inflatable tires and started a new industry. Although the safety bicycle replaced the ordinary, a remnant of the ordinary remains in the anachronism of expressing gearing in terms of the equivalent diameter of the front wheel of an ordinary. It is kept because it provides a way to express gearing independently of the particular implementation on a bicycle.
The early safety bicycles had a single gear and, by today's standards, were geared quite low, making the bicycles slow at ordinary pedalling speeds. After World War II, multi-speed rear hubs and integral coaster brakes provided greater versatility and increased safety for the ordinary cyclist. Shifting gears on a multi-speed rear hub is typically done with a shift mechanism mounted on the handlebars and connected to the rear hub by a cable. A shift lever on the mechanism is moved a pre-set distance and the cable, adjusted to a precise length, moves a pin in the hub the correct amount to effect the gear change. A more rugged mechanism, the derailleur, uses a cluster of several chain sprockets of different diameter and a mechanism which transfers the chain from one sprocket to another, actuated by a cable connected to a shift lever.
Just as for cars with manual transmissions, many people find the shift mechanisms on bicycles inconvenient and confusing. Despite a need for an automatic transmission for a bicycle, none of the many mechanisms proposed have been commercially acceptable. Many are simply too bulky and heavy. Some are not truly automatic, e.g. requiring back pedalling to cause a shift. Others are too expensive relative to the cost of the rest of the bicycle and still others are not sufficiently durable.
In the prior art, a variety of automatic transmissions have been proposed for bicycles. U.S. Pat. No. 3,613,466--Houghton--discloses a mechanism for automatically shifting a derailleur mechanism in response to tension in the chain. U.S. Pat. No. 3,513,725--Shimano--discloses a planetary gear system connected to a ratchet and pawl mechanism in which the pawls are moved by centrifugal force. U.S. Pat. No. 4,598,920--Dutil--discloses a variable diameter rear sprocket in which sprocket teeth are moved radially in response to the rotational speed of the wheel. U.S. Pat. No. 2,165,201--Baumgartner--discloses a foot operated transmission connected to the pedal crank of a bicycle and having a plurality of pairs of constant mesh gears selected by a cam controlled by back pedalling a fraction of a revolution to select the desired pair of gears.
To be commercially viable, an automatic transmission for a bicycle has to be rugged, yet light, compact, and easily made and assembled. This is particularly true when the transmission is to be attached to the rear wheel. Connecting the transmission to the rear wheel has the advantage of not requiring a special bicycle frame, as required, for example, by the transmission disclosed in the Baumgartner patent. To fit with a wheel hub inside the rear fork of a bicycle, a transmission must be narrow, e.g. no more than one inch (2.5 cm.) thick, measured from side to side as mounted on a bicycle. The narrowness of the transmission places a premium on lateral space within the transmission. Pawls, which provide the mechanical coupling between the gears and the hub, must be as wide as possible for strength yet not interfere with one another. The pawls must also be accurately located since tolerances add to the width of the transmission, reducing the number of possible gears.
For ease of manufacture and assembly, there should be as few components, and as few different components, as possible. This not only aids the manufacturer but also reduces spare parts inventories for bicycle shops.
In view of the foregoing, it is therefore an object of the invention to provide an improved automatic transmission for bicycles.
Another object of the invention is to provide a constant mesh, automatic transmission which can be attached to the rear wheel of bicycles.
A further object of the invention is to provide an automatic transmission for bicycles which can be reconfigured to different gear ratios by re-arranging the gears.
Another object of the invention is to provide an automatic transmission for bicycles having constant mesh gears in which the gears on the countershaft can be interchanged with gears on the drive shaft.
A further object of the invention is to provide an automatic transmission for bicycles having constant mesh gears in which the gears on the countershaft are the same as the gears on the drive shaft.